# CBSE Class 12th Physics Notes: Atoms

Mayank UttamCBSE chapter wise notes of class 12 Physics based on chapter 12, Atoms of NCERT textbook are available in this article. These notes are based on latest CBSE syllabus 2017 and are very important for CBSE Class 12 Physics board exam 2017.

The topics covered in these key notes are given below

Alpha-Particle Scattering and Rutherford’s Nuclear Model of Atom |

Observation of the Alpha (α) Scattering Experiment |

Conclusion from Alpha (α) Scattering Experiment |

Some Important terms related to Alpha (α) Scattering Experiment • Impact Parameter • Angle of Scattering |

Postulates of Bohr Model of the Hydrogen Atom |

Radii of Bohr’s Stationary Orbits |

Velocity of Electron in Bohr’s Stationary Orbits |

Total Energy of Electron in Bohr’s Stationary Orbits |

Energy levels of Hydrogen Atom |

Hydrogen spectrum |

The notes are given below:

**Introduction**

In 1898, J. J. Thomson in proposed first model of atom. According to this model, the +ve charge of the atom is uniformly distributed throughout the volume of the atom and the negatively charged electrons are embedded in it like seeds in a watermelon. This model is also known as plum pudding model of the atom. This model was able to explain the overall neutrality of an atom. However, its assumption that the total mass of an atom is uniformly distributed all over the atom was inconsistent with some of the experimental results.

After some years, Rutherford proposed a new model of atom based on Alpha-Particle Scattering experiment.

**CBSE Syllabus for Class 12 Physics 2017**

**Alpha-Particle Scattering and Rutherford’s Nuclear Model of Atom**

**
**

In 1911, Ernst Rutherford suggested some experiments to H. Geiger and E. Marsden.

Both scientists performed an experiment whose schematic diagram is shown in the figure given above.

They directed a beam of 5.5 MeV α-particles emitted from a ^{214}Bi_{83} radioactive source at a thin metallic foil (made of gold).

The beam was allowed to fall on a thin foil of gold of thickness 2.1 × 10^{‒7} m.

The scattered alpha-particles were observed through a rotatable detector consisting of zinc sulphide screen and a microscope. The scattered alpha-particles on striking the screen produced brief light flashes or scintillations. These flashes may be viewed through a microscope and the distribution of the number of scattered particles may be studied as a function of angle of scattering.

**Observation of the Alpha (α) Scattering Experiment **

Many of the α-particles pass through the foil which means that they do not suffer any collisions.

Approximately 0.14% of the incidents α-particles scatter by more than 1 º and about 1 in 8000 deflect by more than 90º.

**Conclusion from Alpha (α) Scattering Experiment **

Based on α scattering experiment, Rutherford concluded the following important points.

According to him

• Scattering of alpha particles is due to columbic force between positive charge of α particle and positive charge of atom.

• Rutherford’s experiments suggested the size of the nucleus to be about 10–15 m to 10–14 m.

• The electrons are present at a distance of about 10,000 to 100,000 times the size of the nucleus itself.

• Atom has a lot of empty space and the entire mass of the atom is confined to very small central core also known as nucleus.

**Some Important terms related to Alpha (α) Scattering Experiment**

**
**

*Impact Parameter*

It is the perpendicular distance of the velocity vector of α particle from the central line of the nucleus, when the particle is far away from the nucleus of the atom.

*Angle of Scattering*

It is the angle made by α particle when it gets deviated from its original path around the nucleus.

**Postulates of Bohr Model of the Hydrogen Atom**

(1) Bohr’s first postulate was that an electron in an atom could revolve in certain stable orbits without the emission of radiant energy, contrary to the predictions of electromagnetic theory. According to this postulate, each atom has certain definite stable states in which it can exist, and each possible state has definite total energy. These are called the stationary states of the atom.

(2) Bohr’s second postulate defines these stable orbits. This postulate states that the *electron *revolves around the nucleus *only in those* *orbits for which the angular momentum is some integral multiple of* *h/*2π where *h *is the Planck’s constant (= 6.6 × 10–34 J s). Thus the angular momentum (*L*) of the orbiting electron is quantised. That is, *L _{n} = nh/2*π

(3) Bohr’s third postulate incorporated into atomic theory the early quantum concepts that had been developed by Planck and Einstein. It states that *an electron might make a transition from one of its* *specified *non-radiating orbits to another of lower energy. When it does so, a photon is emitted having energy equal to the energy difference between the initial and final states. The frequency of the emitted photon is then given by, *h v = E _{i}* ‒

*E*

_{f}Where, *E _{i}* and

*E*are the energies of the initial and final states and

_{f}*E*>

_{i}*E*.

_{f}**Radii of Bohr’s Stationary Orbits**

The radius is given by,

From the above equation, we can see that *r* ∝ *n*^{2}(here Z is atomic number). It means radii of stationary orbits are in the ratio 1^{2 }: 2^{2} : 3^{3}… and so on. So, stationary orbits are not equally spaced.

**Velocity of Electron in** **Bohr’s Stationary Orbits**

The velocity is given by,

This means *v* ∝ (1/*n*).

**Total Energy of Electron in** **Bohr’s Stationary Orbits**

Total energy is given by,

The negative sign of the total energy of an electron moving in an orbit means that the electron is bound with the nucleus. Energy will thus be required to remove the electron from the hydrogen atom to a distance infinitely far away from its nucleus (or proton in hydrogen atom).

Note: Bohr model is valid for only one-electron atoms/ions; an energy value, assigned to each orbit, depends on the principal quantum number *n *in this model*. *We know that energy associated with a stationary state of an electron depends on *n *only, for one-electron atoms/ions. For a multi-electron atom/ion, this is not valid.

**Energy levels of Hydrogen Atom**

The total energy of an electron in *n*^{th} orbit of a hydrogen atom is given by *E _{n}* = (‒13.6/

*n*

^{2}) eV.

Putting *n* = 1, 2, 3… is above equation, we get the energies of electron as, *E*_{1} = ‒ 13.6 eV, *E*_{2} = ‒ 3.4 eV, *E*_{3} = ‒1.51 eV…

These energy levels of hydrogen atom are represented in energy level diagram given below.

**Hydrogen spectrum**

Atoms of each element are stable and emit characteristic spectrum. The spectrum consists of a set of isolated parallel lines termed as line spectrum. It provides useful information about the atomic structure.

The atomic hydrogen emits a line spectrum consisting of various series.

The frequency of any line in a series can be expressed as a difference of two terms